The present invention relates to sensors for determining the concentration of a constituent of a fluid stream, such as an oxygen sensor for monitoring of atmosphere in industrial processes. In the present invention, apertures, formed in a three-fold symmetrical arrangement, may be located in a conductive protective sheath to impede laminar flow near the tip of the sensor element, thus improving the accuracy of the sensor. In addition, the sensor tip may, without modification, allow the sensor to accommodate more than one type of sensor element.
Solid-state electrolyte sensors are commonly used for monitoring and control of a number of different industrial atmospheres; one example is an oxygen sensor. Although termed xe2x80x9coxygen sensorsxe2x80x9d, these devices are actually sensitive to a number of variables. In highly reducing atmospheres (very low oxygen with chemical species present that readily react or xe2x80x9creducexe2x80x9d oxygen), these types of sensors do not, in fact, measure oxygen levels. Instead, under such conditions, they are a measure of the reduction potential of the gas. In other words, they measure the rate at which the atmosphere can reduce oxygen ions that transit the zirconium oxide cell.
To further complicate the situation, these sensors are also highly sensitive to kinetic elements of the atmosphere. When a gas moves quickly it can more efficiently remove and reduce oxygen ions even though its composition does not change. In many furnaces where such probes are used, the high volume of gas flow in the furnace creates laminar flow over the tip of the sensor. This flow can result in false low readings of oxygen or reduction potential (or carbon potential as it is commonly referred to in the heat-treating market). Conventional sensor designs expose the tip of the substrate via a number of holes or slots cut in a protection tube most commonly fabricated of stainless steel (RA330 is most common). This type of xe2x80x9cventilationxe2x80x9d is considered very important in avoiding a build up of oxygen inside the protective sheath. Unfortunately, the geometry employed for this xe2x80x9cventilationxe2x80x9d allows laminar flows to travel into and through the probe, negatively affecting accuracy.
Additionally no sensor tip design, to date, has been able to accommodate more than one type of sensor element. As there are three standard types of zirconia substrates commonly used in oxygen sensors, as well as new element designs currently under development, a need exists for a single sensor tip design which will readily accept more than one type of sensor element.
Thus, there is a need for a sensor design that may impede laminar flow travel into and through the probe, and for a sensor tip that may accommodate more than one type of sensor element, particularly one which may accept any of the three standard types of zirconia substrates commonly used in oxygen sensors.
Accordingly, it is an object of the present invention to obviate problems of the prior art. It is a related object of the present invention to provide a sensor that impedes laminar flow and promotes turbulent flow over the tip of the sensor element. It is an additional object of the present invention to provide a sensor that may accommodate a broad range of zirconium oxide sensor elements (also referred to as xe2x80x9csubstratesxe2x80x9d) without requiring modification or additional parts.
The design of the present invention includes a unique, three-fold symmetry for ventilation that impedes laminar flow and promotes turbulent flow over the tip of the sensor element. As a result, this design is significantly more accurate in environments where laminar flow of gases is found. If flow patterns result in abnormally high kinetic contribution to the sensor signal on one side of the probe, the lack of through-flow in a sensor of the present invention results in a comparable reduction of kinetic contribution on the opposite side of the probe.
A further innovation of the new tip design is a internal geometry that readily accepts any of the three standard types of zirconia substrates commonly used in oxygen sensors as well as new element designs currently under development. To date, no sensor tip design has been able to accommodate more than one type of sensor element.